The invention is directed to a method and an apparatus for thermally treating an oil reservoir, in particular to a method and an apparatus for thermally treating an oil reservoir by separately introducing a first and a second composition into an oil well and contacting the first and second compositions in a pay zone of the oil reservoir to initiate a chemical reaction producing heat and gases.
The method and apparatus according to the present invention are especially useful for extracting petroleum (in the following also named as “crude oil” or simply “oil”) from water-flooded oil reservoirs.
The extraction of petroleum from an oil reservoir usually starts with recovery methods using underground pressure in the oil reservoir which will force the oil to the surface. Over the lifetime of the oil well the pressure decreases, and it becomes necessary to use other extraction methods such as using pumps or injecting water, natural gas or other gases into the oil well to bring the oil to the surface. After these recovery methods are no longer effective the oil reservoir usually still contains considerable amounts of petroleum being enclosed in small cavities or pores of the rock or sand formations.
To recover also these remaining amounts of petroleum tertiary oil recovery methods are used which mainly have the aim to reduce the viscosity of the petroleum. One common method is to inject hot steam into the oil well to heat the petroleum and thus to reduce its viscosity. This method, however, is efficient only up to a depth of about 1 km as otherwise the hot steam will be cooled down before reaching the pay zone. Further, with this method up to 3 to 5 months of injecting hot steam are necessary to heat up the pay zone. For producing the hot steam up to 20 to 30% of the extracted oil is used so that the efficiency of such method is limited.
The injection of hot steam into the oil well is for instance used in the SAGD method (steam assisted gravity drainage). This method is especially used in Canada to extract oil from Canadian oil sands. In the SAGD method, two parallel horizontal oil wells in a length of up to about 200 m are drilled in the pay zone of the oil reservoir, one about 4 to 6 meters above the other. In the upper well hot steam is injected, and the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam is collected by the lower one of the horizontal oil wells and pumped to the surface. As a result of the temperature increase in the heated area of the oil reservoir the viscosity of heavy crude oil or bitumen is reduced which allows it to flow down into the lower wellbore. Further fractures may be generated in the formation as a result of differential thermal expansion what further improves the flow of oil to the lower wellbore.
One of the major disadvantages of the SAGD method are the high costs for producing the hot steam. Up to about 20 to 30% of the extracted oil are needed for its production. Further, as already mentioned above, the injection of hot steam is efficient only up to a depth of about 1 km as otherwise the hot steam will be cooled down before reaching the pay zone. In addition, large amount of water and large water recycling facilities are needed, wherein the availability of water is sometimes a constraining factor.
Alternatively, surfactants or solvents can be injected into the oil well to leach out the petroleum. These methods however, have the disadvantages that the extracted petroleum will be contaminated by these chemicals so that additional efforts and costs are necessary to recover the petroleum.
A further tertiary oil recovery method is characterized in that a chemical reaction is initiated in the pay zone of the oil reservoir to produce hot gases which heat up the oil in the pay zone to reduce its viscosity and to support oil recovery by increasing the pressure in the oil well.
In the Russian patent applications RU 2 100 583 C1, RU 2 126 084 C1 and RU 2 153 065 C1 are disclosed fuel and oxidizing compositions (FOC) being able to produce hot gases after initiating a chemical reaction. These compositions are intended to be introduced into the oil well of an oil reservoir for a thermochemical treatment of the pay zone. These chemical compositions are aqueous solutions containing large amounts of up to 60% by mass or more of ammonium nitrate, NH4NO3. The other components of these FOC are for instance glycerin, nitric acid, carbamide, potassium permanganate, acetic acid, isopropyl metacarborane and acetylsalicylate. After injection of the FOC into the oil well it is ignited by initiating a fuse explosion. The decomposition of 1 kg of FOC results in emitting a quantity of heat of about 500-1000 kcal.
These FOC contain an excess of oxygen and hence have a substantial oxidizing character, so that with the admixture of petroleum an explosive composition is created. Further, aqueous solutions containing large amounts of ammonium nitrate are explosive if the water content is below a critical amount of about 16-18% by mass. Hence, in view of a safety handling of such compositions the water content is usually above 26-28% by mass. However, with increasing water contents it becomes more and more difficult to achieve a stable reaction with a high output of heat.
In the RU 2 194 156 C1 the FOC contains mainly the reaction product of nitric acid with an alkanolamine, alkyl amine or alkyl polyamine and up to 2.0 to 35.0% by mass of an inorganic nitrate such as ammonium nitrate, potassium nitrate, sodium nitrate or calcium nitrate. With such composition a more safety handling was achieved as the amount of ammonium nitrate could be reduced substantially. However, with the usual way of igniting the FOC by means of a fuse explosion, for safety reasons a mass of maximum 1 to 2 tons can be ignited only. After decomposition of the FOC feed with a mass of 1 to 2 tons the whole operation of the FOC delivery and initiating charge insertion has to be repeated so that at an oil well with a depth of 1 to 2 km not more than about 10 tons of FOC can be reacted per day. If the oil well depth is about 3 to 4 km the amount of FOC to be reacted per day with this method decreases to about 5 tons. Hence, the extent of heating the pay zone and thus the efficacy of this method is limited.
In RU 2 224 103 C1 are described a method and a device for thermochemical treatment of a productive layer. Like in the prior art mentioned before a combustible oxidizing composition comprising ammonium nitrate and water is pumped down into the oil well and is then contacted with an igniting material. The device used with such compositions and described in this document comprises two coaxially arranged pipes for separately introducing the compositions into the oil well. By use of sealing devices an enclosure is formed which encloses the igniting material. At the upper and lower ends of this enclosure shear pins are arranged being able to open the enclosure and allow ignition of the chemical reaction. However, neither the method nor the device described in RU 2 224 103 C1 are efficient when used in water-flooded oil formations as they do not allow to maintain a stable and continuing reaction in the pay zone of the oil reservoir.
In WO 2010/043239 A1, the disclosure of which is hereby incorporated by reference, is described a chemical system of gas evolving oil viscosity diminishing compositions (GEOVDC) for stimulating the productive layer of an oil reservoir for thermally treating an oil reservoir. These compositions are a thermal gas emitting composition (TGEC) and a reaction initiator stabilizer (RIS). By separately introducing these two compositions into the oil well and contacting them in the pay zone of the oil reservoir a chemical reaction is initiated producing heat and evolving gases so that the extraction of oil (petroleum) is improved. This system allows to initiated and maintain a stable and continuous reaction so that the compositions can be continuously introduced even after the chemical reaction was initiated. Up to about 100 tons per day can be reacted so that the efficiency of the oil recovery process is improved.
In the WO 2010/043239 A1 are further described different apparatus for thermally treating an oil reservoir by using this chemical system. However, for providing these apparatus it is often necessary to manufacture specific components what increases the costs for the thermal treatment. Further, when using the methods and apparatus described in this document it is not possible to introduce the compositions into the oil well and to extract the resultant oil or oil containing mixture at the same time through the same oil well. When simultaneous thermochemical treatment and oil extraction is intended it is necessary to use one oil well as an injection well and a second oil well as a producing well what limits the efficiency of this method.
It is therefore the object of the present invention to provide a method and an apparatus for thermally treating an oil reservoir, especially a water-flooded oil reservoir, being simple and cost efficient in use and allowing a flexible control of the steps of introducing and extracting material into or out of the oil well.